Investment casting involves introducing molten metal into molds made from refractory materials, such as ceramics. Slurries containing refractory material and other materials, such as binders, dispersing aids, etc., are formed. A pattern formed from a wax composition is immersed in a first slurry, which deposits refractory material on the pattern's surface. Stucco material is applied to the refractory material. The first such layer applied to the pattern is referred to as the facecoat, and contacts the metal during the casting process. Plural additional layers are then applied to the pattern to form the mold. The pattern is removed from inside the mold by heating the mold/pattern composite. Removing the pattern forms an internal void in the mold having the shape of the desired article. Molten metal is poured into the void and allowed to solidify. The mold is then removed from about the article.
Pattern waxes are commercially available from a number of vendors, Pattern wax compositions used in conventional casting processes typically include significant amounts of filler materials, such as at least 30% filler. Examples of conventional fillers include urea and water. Fillers are added to the wax compositions for a number of reasons, including to reduce the amount of wax used, and to change certain physical properties of the wax composition, such as shrinkage.
Pattern wax and wax fillers generally are partially or totally removed from the mold prior to pouring metal. Wax and wax fillers typically are removed from the mold by first autoclaving the mold/pattern composite, followed by firing under oxygen-rich environments to remove any remaining residues. Certain known fillers, such as acrylates, are difficult to burn completely.
Moreover, firing operations can result in the release of volatile organic compounds (VOCs). The investment casting industry works under stringent environmental regulations governing the release of VOCs into the atmosphere. Methods commonly used to control atmospheric release of VOCs include after-burners and incinerators. Using these known methods to comply with environmental regulations is both technologically and financially difficult. Furthermore, the Environmental Protection Agency likely will promulgate even more stringent regulations regarding the atmospheric release of VOCs in the near future.
Polyalkylene carbonates have been used in the casting industry, and some inventions using polyalkylene carbonates have been patented. For example, EPO 300 039 B1 (EP '039) concerns ceramic-making compositions that contain polyalkylene carbonate binders, particularly polyethylene and polypropylene carbonate binders. These binders were investigated because they completely decompose, volatilize rapidly at relatively low temperatures, and are nontoxic. EP '039 states that polyalkylene carbonates thermally decompose to form carbon dioxide and water.
Santangelo et al.'s U.S. Pat. No. 4,633,929 (Santangelo) concerns a method for producing metal castings by evaporative pattern casting (sand casting) using patterns made from polyalkylene carbonates. The polyalkylene carbonate material (such as polypropylene carbonate) is shaped into the desired casting pattern. The pattern is optionally coated with a refractory coating, such as a silica/water-based slurry, and then embedded in support material, such as sand. Thermal decomposition of this class of polymer indicates a sharp decomposition into mainly CO.sub.2 and H.sub.2 O products, with almost no trace of residual ash. This results in few, if any, surface flaws on the metal castings, which are prevalent on metal castings made using polystyrenes, polyurethanes or similar organic foams.
Cannarsa et al.'s U.S. Pat. No. 4,773,466 (Cannarsa) concerns a method for preparing a polycarbonate copolymer foam suitable for lost-foam casting. Cannarsa is particularly concerned with making patterns using polyalkylene carbonates made from cyclic alkylene precursors (such as cycloheptene oxide, cyclohexene oxide and cyclopentene oxide), which are coated with a refractory for use as patterns.
Kawachi et al.'s U.S. Pat. No. 4,981,948 (Kawachi) concerns a zinc-containing solid catalyst useful for preparing polyalkylene carbonates. Kawachi states that "polymers produced by the process of the invention have a good transparency and can be completely decomposed by heating. For these properties, they find applications as materials for the production of optical fibers, optical discs, ceramic binders, and lost foam castings in addition to as general purpose shaped structures, including films and fibers." Kawachi, at column 6, lines 6-14. Emphasis added.
Kuphal et al.'s U.S. Pat. No. 4,874,030 concerns blends of polypropylene carbonate and poly(methyl methacrylate). These polymeric blends are used in decomposition molding and as binders for ceramic particles. Kuphal, column 2, lines 54-67.
Quinn et al.'s U.S. Pat. No. 5,176,188 (Quinn) is particularly directed to investment casting processes. Quinn concerns a method and composition for forming investment casting patterns that include thermally collapsible microspheres. Example 1 of Quinn teaches that the wax pattern composition can include polyalkylene carbonates, particularly 5.1 weight percent polypropylene carbonate.
Kramer et al.'s U.S. Pat. Nos. 4,814,370 and 4,882,110 (referred to collectively as Kramer) concern polyalkylene carbonates as binders useful for making and shaping a green body of ceramic powder and producing a formed ceramic body. The method comprises mixing ceramic powders with binders comprising a polyalkylene carbonate, preferably polyethylene or polypropylene carbonates. These materials are used to provide workability and green strength to ceramic structures, and because they completely burn-out during sintering to leave minimal residual ash in the sintered body.
Despite these prior inventions, there still is a need for processes and compositions for making pattern wax compositions for the investment casting industry that reduce emission of VOCs, as well as provide other benefits.